This invention relates to methods and apparatus for distinguishing materials by their thermal conductivity and has particular application in distinguishing between natural and simulated gems by means of a hand held test probe.
Diamond-like gems produced from material other than carbon have found a significant commercial market. Some of these materials, such as cubic zirconia, have optical properties sufficiently similar to natural diamonds that experienced jewelers have difficulty in distinguishing the gem from natural diamond. The jeweler must remove the gem from its mounting to measure hardness and/or density or, with the gem in its mounting, make relatively complex X-ray tests that sometimes take several hours to perform. Because the hardness test requires scratching of the gem and the X-ray test requires equipment not readily available to jewelers, the density test is most commonly used. The density test does not lend itself to quick measurements with the gem left in its mounting.
In copending U.S. Patent Application Ser. No. 885,502, assigned to the assignee of the present invention, Leland Ashman has suggested that natural diamonds can be distinguished from simulated diamonds by means of a thermal conductivity test. This is because natural diamonds have a much higher thermal conductivity than do simulants. Where a simulant is so close in appearance to a natural diamond that it can not be readily distinguished by visual inspection, the thermal conductivity is distinctive.
The thermal conductivity of natural diamonds had previously been tested by Anthony Schorr using an apparatus disclosed in his U.S. Pat. No. 3,611,786. The device disclosed in that patent includes a small spherical diamond tip pressed against a suitably prepared diamond sample with a considerable force in the order of six million pounds per square inch. The probe is heated and the difference in temperature between the probe and a diamond supporting mass is used, along with the radius of a thermal contact area and the heat input rate, to determine the thermal conductivity of the diamond sample. Such a testing apparatus would not be suitable for use by a jeweler because of the size of the apparatus, the high pressure forces required, and the need for a measurement of temperature drop across the diamond sample. The latter requirement necessitates a first temperature sensor in the probe and a second temperature sensor in the diamond supporting mass.
The above disadvantages of the Schorr apparatus were overcome by Leland Ashman in the device disclosed in the above-mentioned application. In that application, method and apparatus are disclosed for quickly detecting the relative thermal conductivity of the material using a thermal probe. The probe contains a thermistor in circuit with a resistance bridge circuit. The thermistor is at the tip of the probe and is connected as a leg of the resistance bridge. While the probe tip is held firmly against the test material, such as an unknown gem, a controlled current pulse is fed to the thermistor to heat it, the probe tip and the gem. Immediately thereafter, while the probe tip is still against the gem, the change in resistance of the thermistor is indicated by the change in the balance of the bridge. This change in resistance is representative of change in temperature. It is compared with readings taken with the same or an associated probe on a reference material of known thermal conductivity to determine the relative thermal conductivity of the test gem. The reference material might be a natural diamond.
The method and apparatus taught in the copending Ashman application gives excellent results in distinguishing diamonds from zirconia and is a significant improvement over prior diamond testing devices. A gem can be distinguished within a few minutes after lightly touching a probe to a still-mounted gem. However, the device does require one or more steps of balancing the resistance bridge circuit. Also, the thermistor material is generally a poor thermal conductor compared to the other materials used in the probe tip. As a result, there may be a significant temperature drop within the thermistor when the reading is taken and that drop may mask the conductivity of the test gem. Such masking makes the device unreliable when used with some gems having greater thermal conductivity than zirconia. Furthermore, reliable results are dependent on the unknown gem and the reference material being at substantially identical thermal conditions. Any condition other than thermal conductivity which affects balance of the bridge circuit gives a false reading as to conductivity.
A principle object of the present invention is to provide a method and apparatus for testing diamond-like gems to determine whether the gems are natural or simulated.
A further object of the invention is to provide such a test apparatus which provides a reliable readout within seconds by merely touching a probe to a still-mounted gem.
The invention has further application to the testing of other materials.